This invention relates generally to the field of bone fixation, wherein rigid bone plates are affixed to weak, fractured or damaged bones to stabilize, secure and strengthen the bone, typically using mechanical fasteners inserted into the bone to attach the plate to the bone.
The use of rigid bone plates to stabilize weak, damaged or fractured bones is well known, the plates having openings or bores through which bone screws or similar mechanical fasteners are inserted, the bone screws being driven into the bone to secure the bone or bone fragments to the plate. The plates are typically made of rigid metal, and often are constructed of material with sufficient malleability to allow the surgeon to bend or twist the plate in order to optimize the shape or contour of the plate for better conformation to the bone surface.
The fixation of bone plates to small and thin bones, such as for example a rib, presents problems not encountered with larger and thicker bones. One significant problem is the lack of sufficient bone material to achieve secure fixation of the plate to the bone, since the size and/or length of the bone screws may need to be reduced. Another problem may arise if the bone material is inherently weak, such that the bone screws may have a tendency to work loose over time. Another problem in some known systems is that often the bone screws are all oriented perpendicularly to the surface of the bone with their axes in parallel, which may result in gradual separation of the bone and plate.
Rib bones are elongated, curved and thin with a relatively thin cortex (the outer layer of the bone), which presents significant challenges for affixation of bone plates. In many cases short mono-cortical bone screws are not suitable and bi-cortical bone screws are utilized in order to provide sufficiently secure the bone plate to the rib bone, the bone screws passing through both the anterior and the posterior portions of the cortex. Obviously it is not beneficial to have the sharp tips of the bone screws passing through the rib and extending form the posterior cortex into the body cavity.
It is an object of this invention to provide a rib fixation system comprising a bone plate and mono-cortical bone screws that addresses the problems set forth above, wherein bone screws passing only through the anterior cortex provide sufficient affixation of the bone plate to the rib, the security of the system being enhanced by providing paired sets of screw-receiving openings, wherein the axis of each of the bores of the openings in a paired set are offset from the axis normal to the surface of the bone plate, and further wherein the offset axis of each opening of the paired set is approximately ten degrees off from the normal axis in the transverse direction toward the longitudinal midline of the bone plate and is also approximately ten degrees off the normal axis in the longitudinal direction, with the offset axis of each opening in a paired set being turned toward the other opening in the paired set.